Power control in telecommunications networks

ABSTRACT

A radio frequency transmitter is power controlled using a controlling system including an integrating controller, together with an inner and an outer control loop. A tracking signal supplied by the inner loop to the integrating controller of the outer loop is used to avoid windup problems when the transmitted power is saturated.

This patent application claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 60/422,939 filed on Oct. 31,2002, and U.S. Provisional Patent Application Ser. No. 60/500,427 filedon Sep. 5, 2003. This application incorporates by reference the entiredisclosure of U.S. Provisional Patent Application Ser. Nos. 60/422,939and 60/500,427.

The present invention relates to power control in telecommunicationsnetworks, and, in particular, in RF mobile telephone networks andsystems.

BACKGROUND OF THE INVENTION

Power control is important in mobile telephone networks, for example,because it is important to obtain desirably high capacity andefficiency, particularly in CDMA systems. The variable that iscontrolled is called quality. Quality of the communication is controlledwith reference to a quality measure such as: BER (Bit Error Rate), FER(Frame Erasure Rate, BLER (Block Error Rate), number of iterations of aturbo decoder, or the average reliability of decision statistics. Below,for the sake of brevity, the quality measurement will be referred to asBLER. It will, however, be readily appreciated that BER or FER, or otherquality measurement could be used.

Usually an integrating controller is provided to achieve a steady stateperformance with zero control error. The control scheme used is cascadecontrol, see for example FIG. 1 of the accompanying drawings. The ideawith cascade control is to make an inner control loop (2) much fasterthan an outer control loop (4). For transmission power control (TPC) theinner loop controls another quality measure such as for example thesignal to interference ratio (SIR). The outer loop sets the SIRreference value SIR_(r) for the inner loop. The goal of the outer loopis to control the SIR reference value to achieve a BLER that is equal tothe BLER reference BLER_(r). To get a control system that in steadystate achieves a BLER that is equal to the BLER reference, anintegrating controller (9), which can be, for example, a PI controller,a PID controller, or a pure integrating controller, can be used. Thecascade controller illustrated in FIG. 1 comprises an inner control loop(2) and an outer control loop (4). Both control loops have an input of areceived signal (y(k)). In the outer control loop 4, the BLER isestimated in a BLER estimation unit (5) and compared with a BLERreference signal. A subtractor (7) calculates the difference between thereference signal and the BLER estimate to supply an input signal to anintegrating controller (9). The integrating controller (9) produces aSIR reference signal.

The SIR reference signal is compared with an SIR estimate from an SIRestimation unit (3) in the inner control loop (2). The differencebetween the SIR reference and the SIR estimate is supplied to afunction, for example a step function (11) for determining a commandu(k) that sets transmission power. More generally, the SIR estimate andthe SIR reference value could both be supplied to a function thatdetermines a command u(k) for setting the transmission power.

A known problem with an integrating controller (such as a PI, PID, orpure integrating controller) is that it becomes unstable if the controlsignal saturates. This problem is often referred to as the windupproblem. Transmission power control (TPC) saturation of the controlsignal corresponds to situations when the maximum (or minimum)transmitter power is used.

The windup problem in the power control algorithms for third generationmobile telephony systems is well known. The specific problem of windupprotection in WCDMA makes several additions to anti-windup schemes usedin other areas necessary.

As is well known, integrating controllers have the nice property ofbeing able to achieve zero control error in steady state. As an exampleof an integrating controller, a continuous time PI-controller is shownin FIG. 2. Discrete time controllers have similar behaviour; see, forexample, Karl Johan {dot over (A)}ström and Tore Hägglund, “PIDControllers: Theory, Design and Tuning”, Instrument Society of America,Research Triangle Park, N.C., second edition, 1995.

A known problem with integrating controllers is that the integrator partturns unstable when the control signal saturates. This instabilityoccurs because feedback from the process is needed to stabilize thecontroller, which is not open loop stable. In the case of transmissionpower control, saturation can occur when maximum (or minimum)transmission power is used. In this situation the transmission power canonly be decreased (or increased in the case of a minimum), which can beseen as open loop operation of the integrator.

As the controller is not open loop stable the controller state (theintegrator, I-part) can start to build up a large state. This usuallyresults in that it takes a long time for the control loop to startfunctioning again after the saturation state is left. This problem isusually referred to as the windup problem.

SUMMARY OF THE PRESENT INVENTION

According to one aspect of the present invention there is provided amethod for controlling a radio frequency (RF) transmitter, the methodcomprising:

using an integrating controller to produce a reference value of a firstquality measure from a first error signal;

producing an estimated value of the first quality measure relating to anactual value of the first quality measure; and

supplying a tracking signal related to the estimated value of the firstquality measure and the reference value of a first quality measure tothe reference integrating controller.

According to another aspect of the present invention, there is provideda controller for controlling a radio frequency (RF) transmitter, themethod comprising:

an integrating controller operable to produce a is reference value of afirst quality measure from a first error signal;

an estimator operable to produce an estimated value of the first qualitymeasure relating to an actual value of the first quality measure; and

a tracking unit operable to supply a tracking signal related to theestimated value of the first quality measure and the reference value ofa first quality measure to the reference integrating controller.

It is emphasised that the term “comprises” or “comprising” is used inthis specification to specify the presence of stated features, integers,steps or components, but does not preclude the addition of one or morefurther features, integers, steps or components, or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a controller for a third generation mobile system;

FIG. 2 illustrates an integrating controller; and

FIG. 3 illustrates a simplified model of a quality control process;

FIG. 4 illustrates a controller according to an exemplary embodiment ofthe present invention; and

FIG. 5 illustrates the PI controller of FIG. 2 with a tracking signalinput.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To explain the present invention, a simplified model for the qualitycontrol process is illustrated in FIG. 3. The SIR control loop ismodelled as a saturation (21) and a delay (23).

The process that maps SIR to BLER is modelled as a static function (25).This is not important for the invention and can be modelled by anysuitable means. A problem is that SIR and BLER cannot be directlymeasured. SIR and BLER can only be estimated. This is indicated in FIG.3 with two estimation noises v_(k) and w_(k). BLER is usually estimatedby evaluating the CRC flags of received blocks for a period of time.

In WCDMA systems, SIR is usually estimated by using so-called pilotsymbols transmitted from the base station. Pilot symbols arepredetermined symbols that are known to both the base station and themobile terminal. By observing how the pilot symbols are received in themobile terminal, the SIR can be estimated. The estimation is split intwo parts, estimation of signal power, and estimation of interferencepower. The signal power is estimated by observing with what power thepilot symbols are received. The interference power is estimated byobserving how large variation that is seen in the received pilotsymbols. The estimated SIR is then calculated as the ratio of the signalpower estimate and the interference power estimate.

When transmission power saturates (i.e. when the maximum or minimumtransmission power is used) the actual and estimated SIR will no longerfollow SIR_(r) (SIR reference). In the tracking approach of the presentinvention the difference between the estimated SIR and SIR_(r) iscalculated and fed back to stabilize the integrating controller. Ifv_(k) is small the difference will be small, except when thetransmission power is saturated.

FIG. 4 is a schematic illustration of a tracking solution according toan exemplary embodiment of the present invention. The components of FIG.3 are shown, namely the saturation (21), delay (23) and the staticmapping function (25). A reference SIR (SIR_(r)) is input to this modelto produce an SIR estimate (SIR_(est)) and a BLER estimate (BLER_(est)).

A reference BLER (BLER_(r)) is supplied via a log function (31) to asubtractor (33). Also supplied the subtractor (33) is the BLER estimate,via a log function (41), so that the subtractor (33) produces an error ein the desired quality measure, the error being equal to the differencebetween the reference BLER and the estimated BLER. The log functions areintroduced to ensure that the control loop behaves in a linear fashion,and is not important for the invention.

The tracking solution is illustrated by elements (35), (37), and (39). Acontroller 35 (C-BLER) receives as one input the error signal e. Thecontroller also receives a tracking signal e_(s). The controllerproduces a signal representing the reference SIR for supply to the SIRcontrol loop. The reference SIR signal is also supplied, via a delayelement 37, to a subtractor 39 which produces the tracking signal bysubtracting the delayed reference SIR signal SIR_(r) from the estimatedSIR signal SIR_(est).

A PI-controller with tracking signal input to the integrator is shown inFIG. 5. As before, a continuous time loop is shown, but a discrete timeversion is easily derived and would have similar behaviour.

FIG. 5 illustrates an exemplary controller 35 in more detail. As can beseen, the controller includes a gain element 43 of gain K which receivesan input e and supplies an output e*K to an adder 44. The error signal eis also supplied to a component 45 having a transfer function K/T_(i)(where T_(i) is the integration time) whose output is supplied to anadder 46. A second input of the adder 46 is provided by the output froma second component 49 having a transfer function 1/T_(t) (where T_(t) isthe tracking time) as supplied with the error signal e_(s). The outputof the adder 46 is integrated by the integrator 47 (1/s) and supplied tothe adder 44. The output of the adder 44 gives the reference SIR signal.It can be seen that the controller 35 provides the following transferfunction as given in equation 1.

$\begin{matrix}{{SIR}_{r} = {{e^{*}K} + {\frac{1}{s}\left( {\frac{e^{*}K}{T_{i}} + \frac{e_{s}}{T_{t}}} \right)}}} & (1)\end{matrix}$

An alternative implementation would be to use the estimated trackingsignal e_(s) to do “conditional integration”. In such an implementationthe integrator part is not updated if e_(s) is larger than a threshold,i.e. if |e_(s)|>e_(threshold) the integrator is not updated. Thissolution also prevents the integrator state to build up a large value inscenarios of power saturation.

One exemplary implementation of the tracking arrangement includes tofilter e_(s) and use a dead zone. This makes the impact of estimationerrors smaller in the case when power is not saturated. The classicalimplementation of a dead-zone is a block with the following function(input: u, output: y, dead-zone parameter: u_(d)):

$\begin{matrix}{y = \left\{ \begin{matrix}u & {{{if}\mspace{14mu}{u}} \geq u_{d}} \\0 & {{{if}\mspace{14mu}{u}} < u_{d}}\end{matrix} \right.} & (2)\end{matrix}$

The invention is a new application of the tracking approach to thewindup problem. The major improvement compared to existing approachesare that the saturation is estimated by comparing SIR_(r) and SIR_(est)to produce a tracking signal e_s. The invention is applicable totransmission power control systems in both the up-link and thedown-link.

1. A method for controlling a radio frequency (RF) transmitter, themethod comprising: using an integrating controller to produce areference value of a first quality measure from a first error signalwherein the first error signal is based on a reference value of a secondquality measure and an estimated value of the second quality measure,wherein the integrating controller is one of a proportional integrating(PI) controller having the transfer function:${SIR}_{r} = {{e^{*}K} + {\frac{1}{s}\left( {\frac{e^{*}K}{T_{i}} + \frac{e_{s}}{T_{t}}} \right)}}$in which SIR_(r) is the reference value of the first quality measure, eis the error in quality measure, K is a constant, e_(s) is the trackingsignal and T_(i) and T_(t) are time constants relating to theintegration and tracking unit respectively, or a proportionalintegrating derivative (PID) controller; producing an estimated value ofthe first quality measure relating to an actual value of the firstquality measure; and supplying a tracking signal related to theestimated value of the first quality measure and the reference value ofa first quality measure to the integrating controller.
 2. A method forcontrolling a radio frequency (RF) transmitter, the method comprising:using an integrating controller to produce a reference value of a firstquality measure from a first error signal wherein the first error signalis based on a reference value of a second quality measure and anestimated value of the second quality measure; producing an estimatedvalue of the first quality measure relating to an actual value of thefirst quality measure; and supplying a tracking signal related to theestimated value of the first quality measure and the reference value ofa first quality measure to the integrating controller, wherein anadjusted tracking signal is set to zero when the tracking signal iswithin a predefined value range, the adjusted tracking signal beingsupplied to the integrating controller in place of the tracking signal.3. A method as claimed in claim 2, wherein the adjusted tracking signalis set to zero if the absolute value of the tracking signal is less thana predetermined threshold value.
 4. A method for controlling a radiofrequency (RF) transmitter, the method comprising: using an integratingcontroller to produce a reference value of a first quality measure froma first error signal wherein the first error signal is based on areference value of a second quality measure and an estimated value ofthe second quality producing an estimated value of the first qualitymeasure relating to an actual value of the first quality measure; andsupplying a tracking signal related to the estimated value of the firstquality measure and the reference value of a first quality measure tothe integrating controller, wherein the integrating controller isoperable to not update the integrator if the tracking signal indicatesthat an update would not be advisable, and wherein the integratingcontroller is operable to not update the integrator if the trackingsignal indicates that the absolute value of the difference between theestimated value of the first quality measure and the reference value ofthe first quality measure is larger than a threshold.
 5. A controllerfor controlling a radio frequency (RF) transmitter, the methodcomprising: an integrating controller operable to produce a referencevalue of a first quality measure from a first error signal, wherein thefirst error signal is based on a reference value of a second qualitymeasure and an estimated value of the second quality measure, whereinthe integrating controller is one of a proportional integrating (PI)controller, having the transfer function:${SIR}_{r} = {{e^{*}K} + {\frac{1}{s}\left( {\frac{e^{*}K}{T_{i}} + \frac{e_{s}}{T_{t}}} \right)}}$in which SIR_(r) is the reference value of the first quality measure, eis the error in quality measure, K is a constant, e_(s) is the trackingsignal and T_(i) and T_(t) are time constants relating to theintegration and tracking unit respectively, or a proportionalintegrating derivative (PID) controller, an estimator operable toproduce an estimated value of the first quality measure relating to anactual value of the first quality measure; and a tracking unit operableto supply a tracking signal related to the estimated value of the firstquality measure and the reference value of a first quality measure tothe integrating controller.
 6. A controller for controlling a radiofrequency (RF) transmitter, the method comprising: an integratingcontroller operable to produce a reference value of a first qualitymeasure from a first error signal, wherein the first error signal isbased on a reference value of a second quality measure and an estimatedvalue of the second quality measure; an estimator operable to produce anestimated value of the first quality measure relating to an actual valueof the first quality measure; and a tracking unit operable to supply atracking signal related to the estimated value of the first qualitymeasure and the reference value of a first quality measure to theintegrating controller, wherein the tracking unit is operable to producean adjusted tracking signal which is set to zero when the trackingsignal is within a predefined value range, the adjusted tracking signalbeing applied in place of the tracking signal.
 7. A controller forcontrolling a radio frequency (RF) transmitter, the method comprising:an integrating controller operable to produce a reference value of afirst quality measure from a first error signal, wherein the first errorsignal is based on a reference value of a second quality measure and anestimated value of the second quality measure; an estimator operable toproduce an estimated value of the first quality measure relating to anactual value of the first quality measure; and a tracking unit operableto supply a tracking signal related to the estimated value of the firstquality measure and the reference value of a first quality measure tothe integrating controller, wherein the integrating controller isoperable to not update the integrator if the tracking signal indicatesthat an update would not be advisable and wherein the integratingcontroller is operable to not update the integrator of the trackingsignal indicates that the absolute value of the difference between theestimated value of the first quality measure and the reference value ofthe first quality measure is larger than a threshold.
 8. A methodaccording to claim 1, as implemented in a computer program productcomprising code elements adapted to be executed on a computer.